Refined converter API (#2593)

src/HotChocolate/Core/src/Types/Utilities/DefaultTypeConverter.cs

@@ -8,13 +8,10 @@
 
 namespace HotChocolate.Utilities
 {
-    public partial class DefaultTypeConverter
-        : ITypeConverter
+    public partial class DefaultTypeConverter : ITypeConverter
     {
-        private readonly ConcurrentDictionary<Type, ConcurrentDictionary<Type, ChangeType>> _converters =
-            new ConcurrentDictionary<Type, ConcurrentDictionary<Type, ChangeType>>();
-        private readonly List<IChangeTypeProvider> _changeTypeProvider =
-            new List<IChangeTypeProvider>();
+        private readonly ConcurrentDictionary<(Type, Type), ChangeType> _converters = new();
+        private readonly List<IChangeTypeProvider> _changeTypeProvider = new();
 
         public DefaultTypeConverter(IEnumerable<IChangeTypeProvider>? providers = null)
         {
@@ -107,11 +104,16 @@ private bool TryConvertInternal(Type from, Type to, object source, out object? c
             Type to,
             [NotNullWhen(true)] out ChangeType? converter)
         {
-            if (TryGetConverter(from, to, out converter)
-                || TryCreateConverterFromFactory(from, to, out converter))
+            if (TryGetConverter(from, to, out converter))
             {
                 return true;
             }
+
+            if (TryCreateConverterFromFactory(from, to, out converter))
+            {
+                return true;
+            }
+
             return false;
         }
 
@@ -127,6 +129,7 @@ private bool TryConvertInternal(Type from, Type to, object source, out object? c
                     if (provider.TryCreateConverter(
                         source, target, TryGetOrCreateConverter, out converter))
                     {
+                        _converters.TryAdd((source, target), converter);
                         return true;
                     }
                 }
@@ -143,16 +146,12 @@ private bool TryConvertInternal(Type from, Type to, object source, out object? c
         {
             if (source == target || target == typeof(object))
             {
-                converter = source => source;
+                converter = s => s;
                 return true;
             }
 
-            if (_converters.TryGetValue(source,
-                out ConcurrentDictionary<Type, ChangeType>? toLookUp)
-                && toLookUp.TryGetValue(target,
-                out ChangeType? changeType))
+            if (_converters.TryGetValue((source, target), out converter))
             {
-                converter = changeType;
                 return true;
             }
 
@@ -173,7 +172,7 @@ private bool TryConvertInternal(Type from, Type to, object source, out object? c
                     return converter((TSource)input);
                 }));
 
-        public static DefaultTypeConverter Default { get; } = new DefaultTypeConverter();
+        public static DefaultTypeConverter Default { get; } = new();
 
         private sealed class DelegateTypeConverter : IChangeTypeProvider
         {
@@ -205,4 +204,4 @@ public DelegateTypeConverter(Type source, Type target, ChangeType converter)
             }
         }
     }
-}
+}

src/HotChocolate/Core/src/Types/Utilities/EnumTypeConverter.cs

@@ -6,9 +6,9 @@ namespace HotChocolate.Utilities
     internal sealed class EnumTypeConverter : IChangeTypeProvider
     {
         public bool TryCreateConverter(
-            Type source, 
-            Type target, 
-            ChangeTypeProvider root, 
+            Type source,
+            Type target,
+            ChangeTypeProvider root,
             [NotNullWhen(true)] out ChangeType converter)
         {
             if (source == typeof(string) && target.IsEnum)
@@ -19,7 +19,7 @@ internal sealed class EnumTypeConverter : IChangeTypeProvider
 
             if (source.IsEnum && target == typeof(string))
             {
-                converter = input => input.ToString();
+                converter = input => input?.ToString();
                 return true;
             }
 

src/HotChocolate/Core/src/Types/Utilities/IChangeTypeProvider.cs

@@ -5,11 +5,39 @@
 
 namespace HotChocolate.Utilities
 {
+    /// <summary>
+    /// A <see cref="IChangeTypeProvider" /> is used by the type converter to create new converter.
+    /// Each <see cref="IChangeTypeProvider" /> can provide one ore multiple value converters.
+    /// </summary>
     public interface IChangeTypeProvider
     {
+        /// <summary>
+        /// Tries to create a converter that can convert a value that is of the
+        /// type <paramref name="source"/> to a value of the type <paramref name="target"/>.
+        /// If this type provider can only handle parts of the conversion it can refer back to the
+        /// root converter to ask other <see cref="IChangeTypeProvider"/> to provide the rest of
+        /// the type conversion.
+        /// </summary>
+        /// <param name="source">
+        /// The source type.
+        /// </param>
+        /// <param name="target">
+        /// The traget type.
+        /// </param>
+        /// <param name="root">
+        /// The root change type provider that has access to
+        /// all registered <see cref="IChangeTypeProvider"/>.
+        /// </param>
+        /// <param name="converter">
+        /// The converter that was produced by this instance.
+        /// </param>
+        /// <returns>
+        /// Returns a boolean indicating if this <see cref="IChangeTypeProvider"/> was able to
+        /// create a type converter.
+        /// </returns>
         bool TryCreateConverter(
-            Type source, 
-            Type target, 
+            Type source,
+            Type target,
             ChangeTypeProvider root,
             [NotNullWhen(true)]out ChangeType? converter);
     }